Coil spring steel

ABSTRACT

a coil spring steel which includes: 0.4 to 0.9 wt % of carbon (C); 1.3 to 2.3 wt % of silicon (Si), 0.5 to 1.2 wt % of manganese (Mn); 0.1 to 0.5 wt % of molybdenum (Mo); 0.05 to 0.80 wt % of nickel (Ni); 0.05 to 0.50 wt % of vanadium (V); 0.01 to 0.50 wt % of niobium (Nb); 0.05 to 0.30 wt % of titanium (Ti); 0.6 to 1.2 wt % of chromium (Cr); 0.0001 to 0.3 wt % of aluminum (Al); less than or equal to 0.3 wt % but greater than 0% of copper (Cu); less than or equal to 0.3 wt % but greater than 0% of nitrogen (N); 0.0001 to 0.0030 wt % of oxygen (O); and a balance of iron (Fe) and unavoidable impurities.

CROSS REFERENCE TO RELATED APPLICTTiON

The present appli cation claims the benefit of priority to Korean Patent Application No. 10-2015-0171960 filed on Dec. 4, 2015, the entire content of which is incorporated herein for all purposes by this reference.

TECHNICAL FIELD

The present disclosure relates to a coil spring steel having improved strength and fatigue life through carbide control.

BACKGROUND

A vehicle generally uses a high strength coil spring having a spring constant of 120 kg/s², and ently, a high strength coil spring having a spring constant of 130 kg/s² is currently being developed. As a coil spring having a higher spring constant between 110 kg/s² to 130 kg/s² is used, a total weight of the vehicle can be reduced by reducing a spring diameter and the number of turns for winding the high strength spring coil, whereas sensitivity of the high strength spring coil increases due to corrosion after chipping and painting separation processes. In addition, since design margins are not secured for vehicles due to the reductionof the coil spring diameter, strength of the coil spring may decrease and a damage rate may increase,

In order to reduce these risks, dual coating and painting processes are applied to prevent corrosion. However, this method may cause an excessive increase in material costs. Accordingly, in the current auto industries, it is necessary to increase durability of vehicles by improving strength and corrosion matters related to materials. Since recent vehicles have high performance, high power, and high efficiency, it is necessary to increase strength and reduce weight of vehicle parts. For example, when steel materials are used for suspensions, the suspensions must be lightweight under heavy load and corrosion conditions, and therefore, the strength and durability of the materials must be necessarily secured.

The foregoing is intended merely to aid in the understanding of the background of the present disclosure, and is not intended to mean that the present disclosure falls within the purview of the related art that is already known to those skilled in the art.

SUMMARY

The present disclosure has been made keeping in mind the above problems occurring in the related art, and the present disclosure is intended to propose a coil spring steel having improved strength and fatigue life through carbide control.

In accordance with one embodiment in the present disclosure, a coil spring steel includes: 0.4 to 0.9 wt % of carbon (C); 1.3 to 2.3 wt % of silicon (Si); 0.5 to 1.2 wt % of manganese (Mn); 0.1 to 0.5 wt % of molybdenum (Mo); 0.05 to 0.80 wt % of nickel (Ni); 0.05 to 0.50 wt % of vanadium (V); 0.01 to 0.50 wt % of niobium (Nb); 0.05 to 0.30 wt % of titanium (Ti); 0.6 to 1.2 wt % of chromium (Cr); 0.0001 to 0.3 wt % of aluminum (Al); less than or equal to 0.3 wt % but greater than 0% of copper (Cu); less than or equal to 0.3 wt % but greater than 0% of nitrogen (N); 0.0001 to 0.0030 wt % of oxygen (O); and a balance of iron (Fe) and unavoidable impurities.

The coil spring steel may have a tensile strength of 2150 MPa or more and a hardness of 690 HV or more.

A wire rod made of the coil spring steel may have a fatigue life of 280 thousand cycles or more under a condition of a bending moment of 20 kgfm and a load of 100 kgf.

A coil spring single-part made of the coil spring steel may have a complex corrosion fatigue life of 360 thousand cycles or more under a complex corrosion environment in which salt of 50±5 (g/L) is sprayed and to which a bending moment of 20 kgfm and a load of 100 kgf are applied.

As apparent from the above description, the coil spring steel of the present disclosure can have improved strength and. fatigue life by controlling the contents of molybdenum (Mo), vanadium (V), niobium (Nb), titanium (Ti), and chromium (Cr) and generating carbide.

In more detail, the the coil spring steel of the present disclosure can have tensile strength increased by 10% and hardness increased by 17%, compared to existing steels including Fe-1.45Si-0.68Mn-0.71Cr-0.23Ni-0.08V-0.03Ti-0.23Cu-0.035Al-0.55

Thus, it is possible to reduce the weight of the coil spring steel by about 15% and improve fuel efficiency by about 0.04%. In addition, it is possible to improve the fatigue life of the steel by 27% and improve the corrosion resistance and complex corrosion fatigue life thereof by 33%.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a graph illustrating a result of thermodynamic calculation. on mass fractions of components in cementite in a temperature range of 300° C. to 1600° C. according to an embodiment in the present disclosure.

FIG. 2 is a graph illustrating a result of thermodynamic calculation on amounts of all phases in a temperature range of 300° C. to 1600° C. according to an embodiment in the present disclosure.

DETAILED DESCRIPTION

Exemplary embodiments in the present disclosure will be described below with reference to the accompanying drawings.

A coil spring steel according to an embodiment in the present disclosure includes 0.4 to 0.9 wt % of carbon (C), 1.3 to 2.3 wt % of silicon (Si), 0.5 to 1.2 wt % of manganese (Mn), 0.1 to 0.5 wt % of molybdenum (Mo), 0.05 to 0.80 wt % of nickel (Ni), 0.05 to 0.50 wt % of vanadium (V), 0.01 to 0.50 wt % of niobium (Nb), 0.05 to 0.30 wt % of titanium (Ti), 0.6 to 1.2 wt % of chromium (Cr), 0.0001 to 0.3 wt % of aluminum (Al), less than or equal to 0.3 wt % (not including 0%) of copper (Cu), less than or equal to 0.3 wt % (not including 0%) of nitrogen (N), 0.0001 to 0.0030 wt % of oxygen (O), and a balance of iron (Fe) and unavoidable impurities.

Hereinafter, the limited components of the coil spring steel according to the embodiment in the present disclosure will be described in detail.

[Carbon (C): 0.4 to 0.9 wt %]

Carbon increases strength of steel after quenching the steel. Carbide such as CrC, VC, or MoC is formed during the tempering of the steel. Thus, the steel has improved temper-resistant and softening properties but has low toughness. In order for the carbon to contribute to increasing the temper resistance of the steel and to improving the size invariance and setting property (shape preservation property) of the steel, the carbon allows TiMoC nano-carbide to be formed and heated to a temperature of about 300° C.

When the content of the carbon is less than 0.4 wt %, the strength of the steel insignificantly increases and. the fatigue strength thereof decreases. On the other hand, when the content of the carbon exceeds 0.9 wt %, large infusible carbide is present and the fatigue characteristics and durability life of the steel is deteriorated. In addition, the steel has poor processability before being quenched. Therefore, the content of the carbon is limited to a range of 0.4 to 0.9 wt %.

[Silicon (Si): 1.3 to 2.3 wt %]

Silicon improves elongation percentage of the steel. In addition, the silicon suppresses variation in shape of the steel to improve the setting property thereof, and hardens the ferrite and martensite structures of the steel to increase heat resistance and hardenability thereof.

When the content of the silicon is less than 1.3 wt %, the elongation percentage and setting property of the steel are insignificantly improved. On the other hand, when the content of the silicon exceeds 2.3 wt %, decarburization is caused due to infiltration of oxygen between the carbon and the structure. In addition, the steel has poor processability due to an increase in hardness before being quenched. Therefore, the content of the silicon is limited to a range of 1.3 to 2.3 wt %.

[Manganese (Mn): 0.5 to 1.2 wt %]

Manganese improves the hardenability and strength of the steel. The manganese is solidified in the base of the steel to increases the bending fatigue strength and quenching property thereof. The manganese serves as a deoxidizer for generating oxide and suppresses the formation of inclusions such as aluminum oxide (Al₂O₃). On the other hand, when the manganese is excessively included in the steel, it forms a MnS inclusion and causes high-temperature brittleness.

When the content of the manganese is less than 0.5 wt %, the quenching property of the steel is insignificantly improved. On the other hand, when the content of the manganese exceeds 1.2 wt %, the steel has poor proces ability before being quenched and has a reduced fatigue life due to centerline segregation and precipitation of the MnS inclusion. Therefore, the content of the manganese is limited to a range of 0.5 to 1.2 wt %.

[Molybdenum (Mo): 0.1 to 0.5 wt %]

Molybdenum forms fine precipitates such as TiMoC which is nano-carbide and improves the strength and fracture toughness of the steel.

When the content of the molybdenum is less than 0.1 wt %, the fracture toughness of the steel is insignificantly improved. On the other hand, when the content of the molybdenum exceeds 0.5 wt %, the steel has poor processability and thus has low productivity. Therefore, the content of the molybdenum is limited to a range of 0.1 to 0.5 wt %.

[Nickel (Ni): 0.05 to 0.80 wt %]

Nickel improves the corrosion resistance and heat resistance of the steel and prevents low-temperature brittleness thereof.

When the content of the nickel is less than 0.05 wt %, the corrosion resistance and heat resistance of the steel are insignificantly improved. On the other hand, when the content of the nickel exceeds 0.80 wt %, the steel has high-temperature brittleness. Therefore, the content of the nickel is limited to a range of 0.05 to 0.80 wt %.

[Vanadium (V): 0.05 to 0.50 wt %]

Vanadium forms VC as a fine precipitate, and serves to improve the fracture toughness of the steel. The VO as the fine precipitate suppresses movement of a grain boundary. The vanadium is dissolved and solidified when the steel is austenitized and is precipitated when it is tempered, thereby causing secondary hardening.

When the content of the vanadium is less than 0.05 wt %, the strength and fracture toughness of the steel are insignificantly improved. On the other hand, when the content of the vanadium exceeds 0.50 wt %, the steel has poor processahility, and thus, has low productivity, similarly to the molybdenum. Therefore, the content of the vanadium is limited to a range of 0.05 to 0.50 wt %.

[Niobium (Nb): 0.01 to 0.50 wt %]

Niobium forms fine precipitates, and improves the strength and fracture toughness of the steel. In addition, the niobium refines the structure of the steel and hardens the surface thereof by nitrification.

When the content of the niobium is less than 0.01 wt %, the strength and fracture toughness of the steel are insignificantly improved. On the other hand, when the content of the niobium exceeds 0.50 wt %, the steel has high-temperature brittleness. Therefore, the content of the niobium is limited to a range of 0.01 to 0.50 wt %.

[Titanium (Ti): 0.05 to 0.30 wt %]

Titanium forms fine precipitates such as TiMoC which is nano-carbide, and improves the strength and fracture toughness of the steel. The titanium serves as a deoxidizer, and forms Ti₂O₃ to replace the formation of Al₂O₃.

When the content of the titanium is less than 0.05 wt %, the steel coarsens, and the effect of replacing the formation of Al₂O₃ which is a main cause of fatigue deterioration is small. On the other hand, when the content of the titanium exceeds 0.30 wt %, only the above effect is increased, thereby causing an increase in cost. Therefore, the content of the titanium is limited to a range of 0.05 to 0.20 wt %.

[Chromium (Cr): 0.6 to 1.2 wt %]

Chromium is dissolved in the austenite structure of the steel and forms CrC carbide when the steel is tempered. Accordingly, the chromium improves the hardenability of the steel, accomplishes improvement in strength and grain refinement through suppression of steel softening, and improves the toughness of the steel.

When the content of the chromium is less than 0.6 wt %, the strength and hardenability of the steel are insignificantly improved. On the other hand, when the content of the chromium exceeds 1.2 wt %, only the effect described in the titanium is increased, thereby causing an increase in cost. Therefore, the content of the chromium is limited to a range of 0.6 to 1.2 wt %.

[Aluminum (Al): 0.0001 to 0.3%]

Aluminum improves the strength and impact toughness of the steel. Since the aluminum is added to the steel together with Nb, Ti, and Mo, it is possible to reduce an added amount of vanadium for grain refinement and nickel for securing toughness which are expensive components.

When the content of the aluminum is less than 0.0001 wt %, the strength and impact toughness of the steel are insignificantly improved. On the other hand, when the content of the aluminum exceeds 0.3 wt %, Al₂O₃ which is a large square inclusion is formed, and this acts as a. fatigue origin, thereby deteriorating the durability of the steel. Therefore, the content of the aluminum is limited to a range of 0.0001 to 0.3 wt %.

[Copper (Cu): less than or equal to 0.3 wt % (not including 0%)]

Copper increases the strength of the steel and improves the corrosion resistance thereof as in nickel after the steel is tempered. However, when the content of the copper exceeds 0.3 wt %, alloy costs are increased. Therefore, the content of the copper is limited so as to be less than or equal to 0.3 wt %.

[Nitrogen (N): less than or equal to 0.3 wt % (not including 0%)]

Nitrogen refines crystal grains by reacting with aluminum and titanium and forming AlN and TiN. However, when the content of the nitrogen exceeds 0.3 wt %, the quenching property of the steel may be deteriorated. Therefore, the content of the nitrogen is limited so as to be less than or equal to 0.3 wt %.

[Oxygen (O): 0.0001 to 0.0.30 wt %]

Oxygen is combined with silicon and aluminum to form a hard nonmetallic oxide inclusion, and causes deterioration of fatigue life of the steel. Thus, the content of the oxygen may be low as possible as.

It is currently impossible to limit the content of the oxygen to less than 0.0001 wt %. However, when the content of the oxygen exceeds 0.003 wt %, the oxygen reacts with aluminum to form Al₂O₃, and this acts as a fatigue origin, thereby deteriorating the durability of the steel. Therefore, the content of the oxygen is limited. to a range of 0.0001 to 0.003 wt %.

[Manufacture Method]

A method for manufacturing a coil spring includes processing and filling a steel material, which includes 0.4 to 0.9 wt % of carbon (C), 1.3 to 2.3 wt % of silicon (Si), 0.5 to 1.2 wt % of manganese (Mn), 0.1 to 0.5 wt % of molybdenum (Mo), 0.05 to 0.80 wt. % of nickel (Ni), 0.05 to 0.50 wt % of vanadium (V), 0.01 to 0.50 wt. % of niobium (Nb), 0.05 to 0.30 wt % of titanium (Ti), 0.6 to 1.2 wt % of chromium (Cr), 0.0001 to 0.3 wt % of aluminum (Al), less than or equal to 0.3 wt % (not including 0%) of copper (Cu), less than or equal to 0.3 wt % (not including 0%) of nitrogen (N), 0.0001 to 0.0030 wt % of oxygen (O), and a balance of iron (Fe) and unavoidable impurities, in the form of wire rod.

The coil spring is made in such a manner that a control heat treatment process of maintaining the wire rod at certain high temperature for a certain time and then air cooling the same so as to refine crystal grains and homogenize structures thereof is performed, and quenching and tempering processes of giving strength and toughness to the homogenized wire rod are performed.

[Test Method]

A tensile strength was measured using a standard tensile test specimen having a standard diameter of 4 mm according to KS B 0801, Korean Industrial Standards In addition, the standard tensile test specimen was measured by a 200-ton tester according to KS B 0802.

Hardness was measured at 300 gf using a Micro-Vickers hardnesstester according to KS B 0811.

Wire rod fatigue life was measured using standard tensile test specimen. having a standard diameter of 4 mm according to KS B ISO 1143, using a bending fatigue tester having a maximum bending moment of 20 kgfm and a maximum load of 100 kgf.

A corrosion depth was measured using a complex environmental corrosion tester according to KS D 9502.

Single-part corrosion fatigue life i measured under a salt spray environment.

Complex corrosion fatigue life was measured at a maximum bending moment of 20 kgfm and a maximum load of 100 kgf using a corrosion spring testing machine (CSTM) under a complex corrosion, environment, in which salt of 50±5 (g/L) is sprayed, according to KS D 9502.

EXAMPLES AND COMPARATIVE EXAMPLES

TABLE 1 Wt. % C Si Mn Mo Ni V Nb Ti Cr Al Cu N O Ex. 1 0.62 1.85 0.72 0.29 0.43 0.27 0.18 0.12 0.88 0.006 0.057 0.0018 0.0006 Ex. 2 0.64 1.89 0.78 0.35 0.48 0.30 0.23 0.16 0.92 0.018 0.061 0.0013 0.0009 Ex. 3 0.68 1.93 0.83 0.45 0.55 0.36 0.26 0.21 1.02 0.013 0.042 0.0019 0.0011 Comp 0.71 1.83 0.69 0.08 0.42 0.33 0.25 0.26 0.98 0.004 0.052 0.0015 0.0005 Ex. 1 Comp 0.69 1.84 0.71 0.53 0.49 0.35 0.14 0.18 1.05 0.014 0.065 0.0016 0.0008 Ex. 2 Comp 0.72 1.81 0.86 0.36 0.03 0.29 0.29 0.10 0.99 0.011 0.046 0.0017 0.0012 Ex. 3 Comp. 0.63 1.79 0.62 0.46 0.83 0.28 0.25 0.14 1.10 0.007 0.054 0.0011 0.0009 Ex. 4 Comp 0.58 1.82 0.74 0.32 0.56 0.03 0.19 0.20 0.89 0.014 0.067 0.0015 0.0006 Ex. 5 Comp 0.65 1.78 0.83 0.44 0.51 0.55 0.18 0.23 0.94 0.013 0.043 0.0017 0.0005 Ex. 6 Comp 0.67 1.81 0.71 0.33 0.45 0.25 0.008 0.17 0.95 0.011 0.046 0.0012 0.0010 Ex. 7 Comp 0.66 1.85 0.64 0.38 0.43 0.38 0.53 0.15 0.82 0.008 0.054 0.0011 0.0007 Ex. 8 Comp 0.69 1.86 0.68 0.42 0.48 0.42 0.30 0.04 1.11 0.014 0.067 0.0012 0.0011 Ex. 9 Comp 0.63 1.93 0.74 0.36 0.50 0.37 0.31 0.32 0.99 0.014 0.043 0.0017 0.0014 Ex. 10 Comp 0.58 1.92 0.81 0.39 0.55 0.28 0.25 0.24 0.57 0.006 0.041 0.0013 0.0008 Ex. 11 Comp 0.70 1.83 0.83 0.40 0.54 0.34 0.22 0.25 1.52 0.017 0.040 0.0014 0.0007 Ex. 12

TABLE 2 Single- Wire part Complex rod corrosion corrosion fatigue fatigue fatigue life Corro- life life Tensile Hard- (ten sion (ten (ten strength ness thousand depth thousand thousand (MPa) (HV) cycles) (μm) cycles) cycles) Ex. 1 2150 690 28 18 2.4 36.7 Ex. 2 2180 700 28.5 15 2.6 37.4 Ex. 3 2165 697 28.2 16 2.6 37.6 Comp 2030 620 19 25 1.8 27.4 Ex. 1 Comp 1955 570 21 24 1.7 27.6 Ex. 2 Comp 1850 580 23 26 1.6 27.3 Ex. 1 Comp 1790 590 22 24 1.7 26.9 Ex. 4 Comp 2010 610 21 25 1.8 26.5 Ex. 5 Comp 2025 620 24 27 1.9 29.2 Ex. 6 Comp 2005 630 25 27 2.1 28.2 Ex. 7 Comp 2050 640 21 25 2.0 28.7 Ex. 8 Comp 2090 650 19 23 1.8 27.1 Ex. 9 Comp 1.800 590 18 22 1.9 26.9 Ex. 10 Comp 2080 640 19 23 1.7 25.5 Ex. 11 Comp 1820 600 22 22 2.1 30.3 Ex. 12

Table 1 shows components and the contents thereof according to examples and comparative examples. Table 2 shows tensile strength, hardness, wire rod fatigue life, corrosion depth, single-part corrosion fatigue life, and complex corrosion fatigue life according to examples and comparative examples.

In comparative examples 1 and 2, the contents of other components are controlled in the same range as those of the coil spring steel according to the examples, and only the content of molybdenum (Mo) is controlled so as to be less than or exceed that of the coil spring steel according to the examples.

In the comparative examples 3 and 4, the contents of other components are controlled in the same range as those of the coil spring steel according to the examples, and only the content of nickel (Ni) is controlled so as to be less than or exceed that of the coil spring steel according to the examples.

In the comparative examples 5 and 6, the contents of other components are controlled in the same range as those of the coil spring steel according to the examples, and only the content of vanadium (V) is controlled so as to be less than or exceed that of the coil spring steel according to the examples.

In the comparative examples 7 and 8, the contents of other components are controlled in the same range as those of the coil spring steel according to the examples, and only the content of niobium (Nb) is controlled so as to be less than or exceed that of the coil spring steel according to the examples.

In the comparative examples 9 and 10, the contents of other components are controlled in the same range as those of the coil spring steel according to the examples, and only the content of titanium (Ti) is controlled so as to be less than or exceed that of the coil spring steel according to the examples.

In the comparative examples 11 and 12, the contents of other components are controlled in the same range as those of the coil spring steel according to the examples, and only the content of chromium (Cr) is controlled so as to be less than or exceed that of the coil spring steel according to the examples.

As seen in the table 2, the contents of molybdenum (Mo), nickel (Ni), vanadium (V), niobium (Nb), titanium (Ti), and chromium (Cr) in the comparative examples 1 to 12 do not satisfy the limited content range of the components of the coil spring steel according to the examples. Therefore, it may be seen that the tensile strength and the hardness in the comparative examples 1 to 12 are lower compared to those in the examples 1 to 3.

In addition, it may be seen that the wire rod fatigue life, single-part corrosion fatigue life, and complex corrosion fatigue life in the comparative examples 1 to 12 are poor compared to those in the examples 1 to 3. It may be seen that since the corrosion depth is deeper compared to that in the examples 1 to 3, corrosion performance is lowered.

The molybdenum, vanadium, niobium, titanium, and chromium are components which react with carbon to form carbide, and generate MoC, VC, NbC, TiC, and CrC, respectively. Through such uniform distribution of carbide, the tensile strength and hardness of the steel are increased, and the wire rod fatigue life, single-part corrosion fatigue life, and complex corrosion fatigue life thereof related to durability and corrosion resistance are increased.

These results may be identified through the graph illustrated in FIG. 1. FIG. 1 is a graph illustrating a result of thermodynamic calculation on mass fractions of components in cementite in the temperature range of 300 to 1600° C. in the coil spring steel including Fe-1.6Si-0.7Mn-0.8Cr-0.3Ni-0.3Mo-0.3V-0.1Nb-0.09Ti-0.550 (including small amounts of other Al, Cu, N, and O) according to the present disclosure. lr may be seen that the complex behaviors of eight components are generated for each temperature in the cementite and fine carbides such as MoC, VC, NbC, TiC, and CrC are uniformly distributed.

FIG. 2 is a graph illustrating a result of thermodynamic calculation on amounts of all phases in the temperature range of 300 to 1600° C. in the coil spring steel including Fe-1.6Si-0.7Mn-0.8Cr-0.3Ni-0.3Mo-0.3V-0.1Nb-0.09Ti-0.55C (including small amounts of other Al, Cu, N, and O) according to the present disclosure. It may be seen that various carbides such as MS-ETA and M7C3 in addition to FCC-Al (Austenite), BCC-A2 (Ferrite), and cementite are formed, and the strength and fatigue life of the steel are improved.

As described above, the coil spring steel of the present disclosure can have improved strength and fatigue life by controlling the contents of molybdenum (Mo), vanadium (V), niobium (Nb), titanium (Ti), and chromium (Cr) and generating carbide.

In more detail, the the coil spring steel of the present disclosure can have tensile strength increased by 10% and hardness increased by 17%, compared to existing steels including Fe-1.45Si-0.68Mn-0.71Cr-0.23Ni-0.08V-0.03Ti-0.23Cu-0.035Al-0.55C. Thus, it is possible to reduce the weight of the coil spring steel by about 15% and improve fuel efficiency by about 0.04%. In addition, it is possible to improve the fatigue life of the steel by 27% and improve the corrosion resistance and complex corrosion fatigue life thereof by 33%.

Although the exemplary embodiments have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions, and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 

What is claimed is:
 1. A coil spring steel comprising: 0.4 to 0.9 wt % of carbon (C); 1.3 to 2.3 wt % of silicon (Si); 0.5 to 1.2 wt % of manganese (Mn); 0.1 to 0.5 wt % of molybdenum (Mo); 0.05 to 0.80 wt % of nickel (Ni); 0.5 to 0.50 wt % of vanadium (V); 0.01 to 0.50 wt % of niobium (Nb); 0.05 to 0.30 wt % of titanium (Ti); 0.6 to 1.2 wt % of chromium (Cr); 0.0001 to 0.3 wt % of aluminum (Al); less than or equal to 0.3 wt % but greater than 0% of copper (Cu); less than or equal to 0.3 wt % but greater than 0% of nitrogen (N); 0.0001 to 0 0030 wt % of oxygen (O); and a balance of iron (Fe) and unavoidable impurities.
 2. The coil spring steel according to claim 1, wherein the coil spring steel has a tensile strength of 2150 MPa or more and a hardness of 690 HV or more.
 3. The coil spring steel according to claim 1, wherein a wire rod made of the coil spring steel has a fatigue life 280 thousand cycles or more under a condition of a bending moment of 20 kgfm and a load of 100 kgf.
 4. The coil spring steel according to claim 1, wherein a coil spring single-part made of the coil spring steel has a complex corrosion fatigue life of 360 thousand cycles or more under a complex corrosion environment in which salt of 50±5 (g/L) is sprayed and to which a bending moment of 20 kgfm and a load of 100 kgf are applied. 